Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to nonvolatile memory devices and related methods of programming.
Semiconductor memory devices can be roughly categorized into volatile memory devices and nonvolatile memory devices. Volatile memory devices tend to have rapid read and write speeds, but they lose stored data when disconnected from power. By contrast, nonvolatile memory devices retain stored data even when disconnected from power. Thus, nonvolatile memory devices are commonly used to store data that must be retained even in the absence of power.
Examples of nonvolatile memory devices include, for instance, mask read-only memories (MROMs), programmable read-only memories (PROMs), erasable programmable read-only memories (EPROMs), and electrically erasable programmable read-only memories (EEPROMs).
Unfortunately, MROMs, PROMs, and EPROMs have difficulty updating data because they cannot be freely erased and/or programmed. EEPROMs, on the other hand, can be readily erased and programmed and are therefore used commonly in systems requiring frequent data updates or in auxiliary memory systems.
Flash memory is a form of EEPROM having a relatively high integration density compared with other forms of EEPROM. As a result, flash memory has become widely adopted to provide mass memory storage in auxiliary memory systems. Among the different types of flash memory, there are NAND-type flash memory and NOR-type flash memory, and among these, NAND-type flash memory tends to have the highest integration density.
In many systems, flash memory is repeatedly programmed and erased. Unfortunately, however, there are limits on the number of times that a flash memory can be programmed and/or erased before its memory cells or blocks wear out. Memory cells or blocks may wear out, for instance, through break down of an oxide layer between a control gate and a floating gate of the memory cells. Once a memory cell or blocks wears out, functional degradation and loss of data may occur in the corresponding part of the flash memory.
The lifetime of flash memory blocks is typically measured by a number of erase cycles. For instance, some memory blocks may withstand an average of 100,000 erase cycles before wearing out. Accordingly, to maximize the lifetime of a flash memory device, a conventional flash memory device may distribute erase cycles evenly between different regions of the device.